<?php
$id=$_GET[id_cat];
$con = mysql_connect("localhost","root","admin");
mysql_select_db("achat",$con);
$req = "select * from produit where id_cat='".$id."'";
$data = mysql_query($req);

?>
<select name="liste2">
<?php
while ($row= mysql_fetch_array($data))
{
	echo "<option >".$row['nom_prod']."</option>";
}
?>
</select>